1. Field of the Invention
The present invention relates to selective catalytic reduction and more particularly to systems for converting NO.sub.x to N.sub.2 and O.sub.2 in oxygen-rich environments.
2. Description of Related Art
The control of NO.sub.x emissions from vehicles is a worldwide environmental problem. Gasoline engine vehicles can use newly developed three-way catalysts to control such emissions, because their exhaust gases lack oxygen. But so-called "lean-burn" gas engines, and diesel engines too, have so much oxygen in their exhausts that conventional catalytic systems are effectively disabled. Lean-burn, high air-to-fuel ratio, engines are certain to become more important in meeting the mandated fuel economy requirements of next-generation vehicles. Fuel economy is improved since operating an engine stoichiometrically lean improves the combustion efficiency and power output. But excessive oxygen in lean-burn engine exhausts can inhibit NO.sub.x removal in conventional three-way catalytic converters. An effective and durable catalyst for controlling NO.sub.x emissions under net oxidizing conditions is also critical for diesel engines.
According to a report published February 1992 by the U.S. Environmental Protection Agency, (Office of Air and Radiation, Office of Air Quality Planning and Standards, Research Triangle Park, N.C. 27711), there are, in general, four approaches to controlling NO.sub.x emissions from combustion sources. For example, controlling NO.sub.x formation by modifying the combustion operating conditions, by modifying the combustion equipment, by fuel switching, and by postcombustion control of NO.sub.x by flue gas treatment. The first three approaches reduce the original formation of NO.sub.x. The latter converts the NO.sub.x that was formed to something more benign.
NO.sub.x can be formed during combustion by three fundamentally different mechanisms, e.g., thermal NO.sub.x, fuel NO.sub.x, and prompt NO.sub.x. Thermal NO.sub.x results from the thermal fixation of molecular nitrogen and oxygen in the combustion air, and its formation is extremely sensitive to the local flame temperature and, to a lesser extent, to the local oxygen concentrations. Virtually all thermal NO.sub.x is formed in the region of the flame at the highest temperature. Maximum thermal NO.sub.x production occurs at a slightly lean fuel-to-air ratio due to the excess availability of oxygen for reaction within the hot flame zone. Control of local flame fuel-to-air ratio is critical in achieving reductions in thermal NO.sub.x.
In general, the control mechanisms available for reducing the formation of thermal NO.sub.x include reducing the local nitrogen concentrations at peak temperature, reducing the local oxygen concentrations at peak temperature, reduction of the residence time at peak temperature, and reducing the peak temperature itself.
Fuel NO.sub.x derives from the oxidation of organically bound nitrogen in fuels such as coal and heavy oil, and its formation rate is strongly affected by the rate of mixing of the fuel and air, in general, and by the local oxygen concentration in particular. Typically, the flue gas NO.sub.x concentration from the oxidation of fuel nitrogen is a fraction of the level that would result from complete oxidation of all nitrogen in the fuel. Although fuel NO.sub.x emissions tend to increase with increasing fuel nitrogen content, the emission's increase is not proportional. Fuel NO.sub.x and thermal NO.sub.x formation are each dominated by the local combustion conditions. Fuel-bound nitrogen occurs in coal and petroleum fuels, and the nitrogen-containing compounds in petroleum tend to concentrate in the heavy resin and asphalt fractions during distillation. Little or no fuel NO.sub.x formation is observed when burning natural gas and distillate oil. Reducing fuel NO.sub.x formation for high nitrogen fuels involves introducing the fuel with a sub-stoichiometric amount of air, e.g., a "rich" fuel-to-air ratio. Fuel-bound nitrogen is released in a reducing atmosphere as molecular nitrogen (N.sub.2), rather than being oxidized to NO.sub.x. The balance of the combustion air enters above or around the rich flame in order to complete combustion. Therefore, controlling excess oxygen is an important part of controlling NO.sub.x formation.
Prompt NO.sub.x is produced after first forming an intermediate hydrogen cyanide (HCN) via the reaction of nitrogen radicals and hydrocarbons in the fuel and then by oxidation of HCN to NO. The formation of prompt NO.sub.x has a weak temperature dependence and a short lifetime of several microseconds and is only significant in very fuel-rich flames which are inherently low-NO.sub.x emitters.
The rates of formation of both thermal and fuel NO.sub.x are dependent on the combustion conditions, so modifications of combustion operating conditions can have a substantial impact on the formation of NO.sub.x. The combustion conditions can be tuned by lowering excess air and adjusting the burner settings and air distribution. Overfire air ports, flue gas recirculation systems, and/or low-NO.sub.x burners are also conventional.
Conventional combustion processes typically input some excess air to ensure that all fuel molecules will be oxidized. Low excess air (LEA) systems supply less air than normal to a combustor such that the lower oxygen concentration in the burner zone will reduce the fuel nitrogen conversion to NO.sub.x. Fuel-bound nitrogen is converted to N.sub.2, thus reducing formation of fuel NO.sub.x. The approach is limited by increased carbon monoxide and smoke emissions and reductions in flame stability. Adjusting air registers, fuel injector positions, and overfire air dampers can be used to reduce the minimum excess air level possible. LEA controls require closer operator attention to ensure safe operation; and continuous LEA operations require the use of continuous oxygen and carbon monoxide monitoring, air and fuel flow controls, and instrumentation for adjusting the air flow at various loads.
Off-stoichiometric, or staged combustion, methods create a fuel-rich combustion zone for the initial combustion. The combustion is then completed at lower temperatures in a second, fuel-lean zone. Sub-stoichiometric oxygen is introduced with the primary combustion air into a high temperature, fuel-rich zone to reduce the fuel and thermal NO.sub.x formation. Because the combustion in the secondary zone is conducted at a lower temperature, the thermal NO.sub.x formation is reduced.
Flue gas recirculation (FGR) and exhaust gas recirculation (EGR) recycle a portion of flue gas back to the primary combustion zone to reduce NO.sub.x formation two ways. Heating the inert recycled flue gas combustion products in the primary combustion zone lowers the peak flame temperature, thereby reducing thermal NO.sub.x formation. The thermal NO.sub.x formation is reduced by lowering the oxygen concentration in the primary flame zone. The recycled flue gas can be pre-mixed with the combustion air or injected directly into the flame zone. FGR is limited by the decrease in flame temperature that alters the distribution of heat and lowers fuel efficiency, and FGR only reduces thermal NO.sub.x, so the technique is primarily used for natural gas or distillate oil combustion.
Reduced air preheat is limited to equipment with combustion air preheaters and can be implemented by bypassing all or a fraction of the flue gas around the preheater, thereby reducing the combustion air temperature. Reducing the amount of combustion air preheat lowers the primary combustion zone peak temperature, thereby reducing thermal NO.sub.x formation. Because the beneficial effects are limited to the reduction of thermal NO.sub.x, this approach is economically attractive for only natural gas and distillate fuel oil combustion. Although NO.sub.x emissions decrease significantly with reduced combustion air temperature, significant loss in efficiency will occur if flue gas temperatures leaving the stack are increased as a consequence of bypassing the air preheaters. Enlarging the surface area of existing economizers or installation of an economizer in place of an air preheater can be used to partially recover the heat loss.
In-furnace NO.sub.x reduction or staged fuel injection uses the furnace zone, e.g., the post-combustion, preconvection section for reburning. The burner zone products are passed through a secondary flame or fuel-rich combustion process. A fraction of the fuel is diverted for a secondary flame or fuel rich-zone downstream of the primary combustion zone burner. Sufficient air is then supplied to complete the oxidation process.
Steam or water can be injected into the combustion zone to decrease the flame temperature and thereby reduce the thermal NO.sub.x formation by acting as a thermal ballast. So it is important that such ballast reach the primary flame zone, e.g., by mixing the steam or water into the fuel, combustion air, or injecting the steam or water into the combustion chamber.
Catalytic combustion techniques place a special catalyst coating on a solid surface close to a combustion process. Such catalysts accelerate the chemical reactions and substantial burning can be achieved, even at low temperatures, thereby reducing the formation of NO.sub.x. The catalyst serves to sustain the overall combustion process and minimizes stability problems. Catalytic combustion can be effective in reducing NO.sub.x emissions, as well as emissions of carbon monoxide and unburned hydrocarbons.
Conventional catalysts seriously degrade at high temperatures, e.g., above 1000.degree. C. (1830.degree. F.), and have largely been used in gas turbines.
Injection-type engines, such as diesel and many dual-fuel and natural gas engines, can adjust the air-to-fuel ratio for each cylinder. Such engines are usually operated lean; because the combustion is more efficient and results in better fuel economy. As the oxygen availability increases, the capacity of the air and combustion products to absorb heat also increases, reducing the peak temperatures and lowering the NO.sub.x formation. The limiting factor for lean operation is the increased emissions of hydrocarbons that result at the lower temperatures.
Ignition timing retard is a NO.sub.x control technique that is applicable to internal combustion (IC) engines. Ignition in a normally adjusted IC engine is set to occur shortly before the piston reaches its uppermost position (top dead center, or TDC). At TDC, the air or air-fuel mixture is at maximum compression and power output and fuel consumption are optimum. Retarding causes more of the combustion to occur during the expansion stroke, thus lowering peak temperature, pressure, and residence time. Typical retard values range from 2.degree. to 6.degree., depending upon the engine. Beyond these levels fuel consumption increases rapidly, power drops, and misfiring occurs.
Flue gas treatment reduces NO.sub.x in the flue gas downstream of the combustion zone or by treatment in a boiler unit and can be used in combination with other combustion operation or equipment modifications. Flue gas treatment systems are classified as selective or non-selective, depending on whether they selectively reduce NO.sub.x rather than O.sub.2.
Selective catalytic reduction (SCR) systems usually use ammonia to reduce NO.sub.x to N.sub.2. Ammonia diluted with air or steam is injected through a grid into a flue gas stream upstream of a catalyst bed, such as vanadium, titanium, or platinum-based enclosed in a reactor. The ammonia reacts on the catalyst surface with NO.sub.x to form molecular nitrogen and water and is favored by the presence of excess oxygen. The NO.sub.x reduction is primarily dependent on temperature. A given catalyst will exhibit optimum performance within a temperature range of plus or minus 28.degree. C. (50.degree. F.) where flue gas oxygen concentrations exceed one percent. Below this, the catalyst activity is greatly reduced, thus allowing some unreacted ammonia to slip through. Above the optimum temperature range, the ammonia itself will be oxidized to form additional NO.sub.x. Excessive temperatures may also damage the catalyst.
Non-selective catalytic reduction (NSCR) systems reduce NO.sub.x by a using a catalyst to react the carbon monoxide in the flue gas to form N.sub.2 and carbon dioxide. Such catalysts used usually include a mixture of platinum and rhodium. But certain oil additives, e.g., phosphorus and zinc, can result in catalyst poisoning.
Selective non-catalytic reduction (SNCR) systems selectively reduce NO.sub.x without resorting to the use of catalysts. In a system developed by Exxon (Thermal DeNO.sub.x), gaseous ammonia (NH.sub.3) is injected into the air-rich flue gas to reduce NO.sub.x to N.sub.2. In a process developed by the Electric Power Research Institute (NO.sub.x OUT), a urea-type compound or amine salt is injected into the oxygen-rich and/or high temperature convection section of a boiler to promote NO.sub.x reduction. The exact chemical mechanism is not fully understood but involves the decomposition of urea (C(NH.sub.2).sub.2 O) and the reduction of NO by reaction with NH.sub.2. The temperature is used to control the selective reactions in both systems.
Catalysts that have the activity, durability, and temperature window required to effectively remove NO.sub.x from the exhaust of lean-burn engines are unknown. Prior art lean-NO.sub.x catalysts are hydrothermally unstable. A noticeable loss of activity occurs after relatively little use, and even such catalysts only operate over very limited temperature ranges. Conventional catalysts are therefore inadequate for lean-burn operation and ordinary driving conditions.
Catalysts that can effectively decompose NO.sub.x to N.sub.2 and O.sub.2 in oxygen-rich environments have not yet been developed, although it is a subject of considerable research. But see, U.S. Pat. No. 5,208,205, issued May 4, 1993, to Subramanian, et al. An alternative is to use catalysts that selectively reduce NO.sub.x in the presence of a co-reductant, e.g., selective catalytic reduction (SCR) using ammonia as a co-reductant.
Using co-existing hydrocarbons in the exhaust of mobile lean-burn gasoline engines as a co-reductant is a more practical, cost-effective, and environmentally sound approach. The search for effective and durable SCR catalysts that work with hydrocarbon co-reductants in oxygen-rich environments is a high-priority issue in emissions control and the subject of intense investigations by automobile and catalyst companies, and universities, throughout the world.
SCR catalysts that selectively promote the reduction of NO.sub.x under oxygen-rich conditions in the presence of hydrocarbons are known as lean-NO.sub.x catalysts. More than fifty such SCR catalysts are conventionally known to exist. These include a wide assortment of catalysts, reductants, and conditions. Unfortunately, just solving the problem of catalyst activity in an oxygen-rich environment is not enough for practical applications. Like most heterogeneous catalytic processes, the SCR process is susceptible to chemical and/or thermal deactivation. Many lean-NO.sub.x catalysts are too susceptible to water vapor and high temperatures. As an example, the Cu-zeolite catalysts deactivate irreversibly if a certain temperature is exceeded. The deactivation is accelerated by the presence of water vapor in the stream. In addition, water vapor suppresses the NO reduction activity even at lower temperatures.
The problems encountered in lean-NO.sub.x catalysts include lessened activity of the catalyst in the presence of excessive amounts of oxygen, reduced durability of the catalyst in the presence of water, sulfur, and high temperature exposure, and narrow temperature windows in which the catalyst is active. Practical lean-NO.sub.x catalysts must overcome all three problems simultaneously before they can be considered for commercial use.
Lean-burn engine exhausts have an excessive amount of oxygen that renders conventional three-way catalytic converters useless for NO.sub.x removal. The excess oxygen adsorbs preferentially on the precious metal, e.g., Pt, Rh, and Pd, surfaces in the catalyst, and inhibits a chemical reduction of NO.sub.x to N.sub.2 and O.sub.2. A wide variety of catalysts and reductants are known to promote lean-NO.sub.x catalysis, however, all such catalysts have proven to be susceptible to chemical and/or thermal deactivation. Another major hurdle for commercialization of the current lean-NO.sub.x catalysts is the lack of durability in catalysts to the effects of high-temperature water vapor, which is always present in engine exhaust. Conventional lean-NO.sub.x catalysts are hydrothermally unstable and lose activity after only a short operation time.
Some gasoline can contain up to 1200 ppm of organo-sulfur compounds. These convert to SO.sub.2 and SO.sub.3 during combustion. Such SO.sub.2 will adsorb onto the precious metal sites at temperatures below 300.degree. C. and thereby inhibits the catalytic conversions of CO, C.sub.x H.sub.y (hydrocarbons) and NO.sub.x. At higher temperatures with an Al.sub.2 O.sub.3 catalyst carrier, SO.sub.2 is converted to SO.sub.3 to form a large-volume, low-density material, Al.sub.2 (SO.sub.4).sub.3, that alters the catalyst surface area and leads to deactivation. In the prior art, the only solution to this problem offered has been to use of fuels with low sulfur contents.
Another major source of catalyst deactivation is high temperature exposure. This is especially true in automobile catalysts where temperatures close to 1000.degree. C. can exist. The high-temperatures attack both the catalyst precious metal and the catalyst carrier, e.g., gamma alumina (.gamma.-Al.sub.2 O.sub.3). Three-way catalysts are comprised of about 0.1 to 0.15 percent precious metals on a .gamma.-Al.sub.2 O.sub.3 wash coat and use La.sub.2 O.sub.3 and/or BaO for a thermally-stable, high surface area .gamma.-Al.sub.2 O.sub.3. Even though the precious metals in prior art catalysts were initially well dispersed on the .gamma.-Al.sub.2 O.sub.3 carrier, they were subject to significant sintering when exposed to high temperatures. This problem, in turn, led to the incorporation of certain rare earth oxides such as CeO.sub.2 to minimize the sintering rates of such precious metals.
Because of the remarkable success that has been achieved in the use of modifiers for improving the durability of the modern catalytic converters, this same approach is being used in the attempt to improve the durability of lean-NO.sub.x catalysts. Much effort has therefore been devoted to the use of modifiers to improve the stability of lean-NO.sub.x catalysts in the simultaneous presence of water, SO.sub.2, and high temperature exposure. However, the results are still far from being satisfactory.
The U.S. Federal Test Procedure for cold starting gasoline fueled vehicles presents a big challenge for lean-NO.sub.x catalysts due to the low-temperature operation involved. Diesel passenger car applications are similarly challenged by the driving cycle that simulates slow-moving traffic. Both tests require reductions of CO, hydrocarbons, and NO.sub.x at temperatures below 200.degree. C. when located in the under-floor position. Modifications of existing catalyst oxidation technology are successfully being used to address the problem of CO and hydrocarbon emissions, but no prior art solution exists for NO.sub.x.